With the increasing global awareness of problems regarding environmental pollution and global warming caused due to the dramatic increase in demand for energy and the use of fossil fuels, renewable energy has attracted attention as a future energy source. However, renewable energy has a serious difficulty in establishing a power supply plan because stable power supplies are impossible due to a large power variation according to a change in climate environment. As an alternative to solve the above problems, the importance of an energy storage system (ESS) configured to store electric power which is not consumed and supply the electric power when power supplies are required has emerged all over the world.
The ESS may be used for various purposes throughout the power networks spanning from power plants generating electric power to consumers. In this case, the ESS is used to store idle electric power during a light load (night time) condition and utilize the electric power during an overload (day time) condition, thereby optimizing operation of electric power through load leveling. As technology for the ESS, there are various types of technology such as secondary battery technology, super-capacitor technology, flywheel technology, compressed air energy storage, pumping-up power generation, and the like. The secondary battery technology, which has no geographic restrictions and may be installed in various capacities, has attracted the most attention as the technology for the ESS.
Among secondary batteries, a redox flow battery is a system in which an active material in an electrolyte is oxidized/reduced to charge/discharge the battery, that is, an electrochemical storage system configured to directly store chemical energy of the electrolyte as electrical energy. Much research has been currently conducted to develop redox flow batteries as large-capacity secondary batteries because the redox flow batteries may be manufactured with large capacity, have low maintenance and repair costs, are operable at room temperature, and have a characteristic of independently designing capacities and outputs.
Among these, a vanadium redox flow battery using vanadium ions has attention because it has an advantage in that a vanadium active material is not consumed because positive and negative electrodes are charged and discharged with any change in oxidation number while the active material is circulated between the positive and negative electrodes.
However, to commercialize the vanadium redox flow battery, a plan is required to solve a decline in capacity of the batteries caused due to a separator cross-over phenomenon of vanadium ions, generation of hydrogen in a negative electrode, and oxidation of vanadium ions when exposed to air, and the like.